Radio receiving set



Dec. 14, 1937. s w SEELEY ET AL v 2,102,168

RADIO RECEIVING SET Filed Jan. 29, 1932 |l DETECTOR AMPLIFIER OSCI LLATDR 7/ 1' AUDIO oz'rzcron AMPLIFIER FIG. 2

l INVENTORS Jiuarf W Jee/ey Garrard ownjay BY THEIR ATTORNEY MW Patented Dec. 14, 1937 RADIO RECEIVING SET smmw. Seeley and'Garrard Mountjoy, Jack- 7 son, Mich., assignors, by mesne assignments, to 7 .Radio Corporation of America, New York, N. Y., a. corporation of Delaware Application-January 29, 1932, Serial No. 589,596

8 Claims. (01. 250-20) UNITED STATES PATE-NT- OFF-ICE This invention relates to radio=receiving sets and is particularly useful in superheterodyne receiving sets with either audible. or superaudible intermediate frequency.

'Insuperheterodyne receiving'sets the oscillator combines with the signal to produce a superaudible intermediate frequency. There are two signals that will combine with the oscillator frequency. to produce the same beat'or intermediate frequency. If leis the intermediate frequencyv and'Fo the oscillator frequency then these two frequencies are Fo+k and Fo-lc. Therefore two stations transmitting at two frequencies will in-' terfere with each other to an appreciable extent .even though there is a preceding circuit tuned to the desired one of these frequencies.

The complementary or undesired signal is usually called the image frequency.

An object of the'invention is to balance out the effect ofsuch frequency.

Other objects will appear in'the following description, reference being had to the drawing in which:

Fi'g. 1 is a circuit diagram illustrating the principles of the invention. Fig. 2 is the circuitof a modification.

. In the drawing the antenna; l is coupled through primary 2 to secondary 3', tuned by condenser 4 tothe desired signali This tuned cir cult is designated A. The tunedcircuit A is coupled to coil 5 in the grid circuit of. the radio frequency amplifier [5. We have shown by way of example one such amplifier only, but more may be used if desired. The con dens er fi tunes this grid circuit coil to the desired signal frequency.

This tuned .circuitjis designatedlB. Ordinarilyi the constants of .these'two tuned circuits would be identical and'they will be assumed to have this relation in explaining our invention. The

oscillat0r is indicated at 1 and thefirst detector isdesignatedby 8, The intermediate frequency after detection at ,8 may be amplifiedbyjamplifier 9 detected at l0; amplifiedat audio frequency at II and reproduced in the phones I2 or equivalent device.- Since our invention relates tothe a ancingnout t e ase freq cyw "have not shown the details of the circuits after the radio frequency amplifier. These .circuits may be standard and they will be understood by those skilled in theiart The" source of grid and plate voltages I? has also been omitted to simplify the disclosure. 'We balance out-the image frequency by introducinga very small" capacity I3 between 55 theantenna l and. the radio'frequency amplifier grid. We may introduce this elsewhere, for example, directly into the grid circuit of the 'first detector, but it is preferable to introduce it into the grid circuit of the radio frequency amplifier and we have shown the condenser I3 as connected to the grid M of such amplifier.

To show how the condenser brings about the v eliminationof the image frequencies at substan-V tially 'all the frequencies used in the present broadcast'range it will be necessary to make a mathematical analysis as follows;

Let the fo-llowingbe assumedz':

E =the voltage; across primary..2 of the image.

frequency to'be eliminated L1=the inductance of primary 2 L2=the inductance of the secondaries 3 and 5 There are two paths through which the image;

frequency may reach the grid [4 of the amplifier tube. One is through the transformers 2, 3 and 3.5. The other is through the small condenser .Sincethe circuit A is out of tune with the image frequency and the coupling isloose the V secondary of the transformer does'not materially.

affect the'impedance ofthe prima'ry. Therefore the current I in coi1 2 is;

A" A WL ,7 The voltage inducedin coil 3 by Ellis equal to the mutual inductance M1 multiplied by the current I in coil 2. Its value is therefore a'WM1I. Substituting the value of I, the voltage becomes:

a l. .Li

The current of I1 in the circuit A is the voltage divided by the impedance Z of the tuned loop.

The voltage in circuit B is the current I multiplied by the mutual inductance=7WMzI1. Substituting the value of I1, the voltage becomes The current in circuit B is found by dividing this voltage by the impedance of the tuned loop. The impedance of the loop B is by design made the same as that of A. Therefore the current is given by the equation:

The voltage E1 applied to grid 14 is this current multiplied by the impedance .lL W i of the condenser C1. Therefore,

l 2 a (W)2 The denominator of the fraction in the right hand side of this equation is recognizable as the series impedance Z and this has already been reduced to Substituting this value of Z1 in the expression for the voltage on grid l4, we find E W C KC (2 W- K) W C To make E1 balance out E2 the transformer coils will be arranged to make E1 opposed in phase to E2. This may be done by making M and M2 of opposite sign. To find the value of C2 that will make E1 equal E2 their sum is made equal to zero, thus,

When this equation is solved for C2,

Since M1M2 Would be in the order of 10- the right hand expression M1M2W would be negligible compared to the remainder of the denominator.

Therefore the equation becomes,

With the usual constants for a set of the kind described for use in the broadcast range the condenser Cz will be in the order of 10- or .10 micro-microfarad and a condenser of this extremely small capacity will substantially balance out the image signal throughout the broadcast range.

In solving the value of the condenser, any inherent capacity coupling between the circuits A and B was not considered. Such capacity couplings can be made negligible. However, we have found by test that with a slight capacity coupling between the tuned circuits A and B, the required capacity C2 can be made substantially, constant for any frequency in the broadcast band. In practice, it is also preferable to empirically determine the best value of the condenser in the neighborhood of the calculated value.

Since the circuits A and B are tuned to the desired signal and the amount of energy transferred by these circuits when tuned to resonance is in the neighborhood of 40,000 to 50,000 times as great as that transferred at the frequency at which the balance is to take place, the diminution of the desired signal by any energy flowing thru condenser l3 cannot be noticed.

The image frequency may be balanced out by an electromagnetic coupling instead of by the electrostatic coupling. Such an arrangement is shown in Fig. 2. V

The image voltage E1 passing through the circuits A and B and developed across the condenser 6 in this figure is the same as previously given and is as follows:

By developing the equations along the lines previously given it will be found that the image voltage E2 passing through the coupling Mx and developed across the condenser 6 is as follows:

By equating E1 to E2 it will be found that the desired coupling Mx is as follows:

substitute tuned vacuum tube stages for such tuned loops without departing from the spirit of the invention.

While a particular embodiment of the invention has been illustrated in the drawing and described in the specification, the invention is not to be limited thereto as various modifications may be made without departing from the invention set forth in the claims. 7

Having described our invention, what we claim 1. In superheterodyne receiving sets, a source of signal frequency, an oscillator adapted to beat with the signal frequency derived from said source, an inductance-capacity loop tuned to the signal frequency, a detector having means to connect it across points in said loop, there being a magnetic coupling between said source and said loop and a capacitatively reactive circuit coupled between the signal source and said loop having a capacitative reactance value that produces across said points a voltage of the image of said signal frequency equal in value and opposite in phase to the image voltage produced thereacross through said magnetic coupling to prevent the beating action between the oscillator and image frequencies from affecting thedetector.

2. In superheterodyne receiving sets, a source of signal'frequency, a' radio frequency amplifier stage having a circuit tuned to the signal, a detector, a signal selective circuit containing a coil and condenser coupling the signal source and the amplifier, a source of oscillations to beat with the signal and a capacitatively reactive circuit coupled between the signal source and the tuned circuit of the amplifier stage having a capacitative reactance value that produces in the'tu'ned circuit of the amplifier stage a voltage of the image of said signal frequency, equal in value and opposite in phase to the image voltage produced thereacross through said coil, to prevent the beating action between the oscillator and image frequencies from affecting the detector.

3. In superheterodyne receiving sets, a signal source, an oscillator adapted to beat with the signal and image frequencies derived from said source, a circuit tuned to the signal frequency and coupled to said signal source, a second circuit tuned to the signal frequency and coupled to the first mentioned circuit whereby the signalto-image ratio is increased in the second circuit, a detector connected to the last mentioned circuit, a third untuned circuit coupled between the signal source and said second circuit, the capacities, inductances and coup-ling of said circuits being of such value as to cause image voltages of substantially equal value and opposite phase at the junction point of said second and third circuits.

4. In a superheterodyne receiving set having a vacuum tube for receiving input energy, a collector circuit of signal and image frequency energy, a circuit tuned to the signal frequency and coupled to said collector circuit,asecond circuit tuned to the signal frequency and coupled to the firstmentioned tuned circuit, whereby the signal-toimage ratio is increased in the second circuit, said second circuit being connected to the vacuum tube, a third untuned circuit coupled between the signal source and the said second circuit, the capacities, inductances and coupling of said circuits being of such value as to cause image voltages of substantially equal value and opposite phase at the junction point of said second and third circuits.

5. The method of selectively transferring signal currents of one frequency without change in frequency, in the presence of undesired signal currents of a slightly diiferent frequency, which comprises transferring two currents of'each of said frequencies, proportioning the undesired currents as thus transferred to be equal and opposite, and building up by resonance only one of the signal currents to prevent equality and opposition between the two signal currents as thus transferred.

6. An electrical coupling system having two pairs of terminals, one pair being coupled to the other pair by a tunable selective transfer means and separately by a non-selective transfer means, the non-selective means having, at any frequency within given limits, a transferora'tio equal and opposite to that of the selective means, the selective means beingtuned below said frequency by a predetermined constant frequency diiference.

7. In a superheterodyne receiver, the arrangement for coupling an antenna circuit to a vacuum tube having an input circuit, and reducing image interference, which comprises tunable selective transfer means for coupling said circuits, and non-selective transfer means for separately coupling said circuits, the non-selective means having, at any frequency within given limits, a transfer ratio equal and opposite to that of the selective means, the selective means being tuned to one side of said frequency by a predetermined constant frequency difference substantially equal to twice the intermediate frequency of said receiver.

8. In a system of the character described, a coupling circuit comprising a tunable and untuned channel in parallel with each other and having their constants so related as to deliver similar impulses at the output thereof in opposed phase relationship, said tunable channel comprising two tuned circuits in cascade and said untuned channel comprising a capacitative connection from the input of said first tuned circuit to said second tuned circuit, said untuned channel having a transmission characteristic such that it transmits frequencies within the operating range of the tunable channel at a substantially low efficiency of the order of the transmission efficiency of. said tunable circuits for non-resonant frequencies;

STUART W. SEELEY. GARRARD MOUNTJOY. 

